2009-12-29

Opportunity Sol 331 (Spirit Sol 352)

Everything went well over Xmas, and yestersol Jeff and Frank drove to the next spot, maybe ten meters from the broken-off fragment of the heat shield. (We're calling this the "flank" or "flankshield.") The thermal protection team is interested in this piece, so we're going to drive to it over the next couple of sols and MI it.

Because of the surrounding debris, driving in this area continues to have its dangers. We can't drive where we can't see. Generally, this means we're limited to driving no farther than we can see in the NAVCAMs (maybe 15m or 20m). Another constraint is that we can't drive to a point where we don't have imaging, such as the region we currently can't see on the other side of the flankshield. When there are only rocks around, we can make reasonable assumptions about what might exist in areas we can't see, but with artificial hazards, we can't safely assume much of anything. (For example, I'm personally kind of worried about the six springs that were attached to the heat shield. We've found three, maybe four, so far, leaving two or three more for us to discover. You know how it's easy to find the tack you dropped on the carpet? Just walk barefoot across the floor ....) As a result of these constraints, even though the drive to the far side of the flankshield is a fairly short one -- maybe 12m or so total -- it'll take us at least two sols, one to drive to the far side of the flankshield, and another to bump from there into the zone we can't see from here.

Assuming the rover lives that long. Well, maybe that's pessimistic of me, but there is a developing concern about our power situation. Alarmingly, our power went down 13% yestersol and another 17% thisol -- that translates to about 100 Watt-hours each sol, a considerable fraction of the 800 Watt-hours or so that we started with.

At least the cause is known: PANCAM tau (atmospheric opacity) measurements indicate a big dust storm. We have a big meeting about this, which I cover so Frank can work on the drive. Frank and I both hate meetings, so I decide to get some mileage out of this: "You owe me one!" Here's what comes out of this meeting:

On sol 328, we had 740 W-hr and a tau about 0.6; on sol 329, 670 W-hr and a tau about 0.8; on sol 330, 563 W-hr and a tau about 1.2. For reference, there was a dust storm just before we landed, and since then the tau has been less than 0.45 or so. Conservatively, we're going to plan on seeing 20% degradation per sol until we bottom out at ~350 W-hr (which will take only another 2 or 3 sols at that rate). That would correspond to a tau of about 3. The safe assumption is that we have not yet seen the worst.

Art skates a tau plot over to me. It's the whole story in one picture. There's this nice, steady decline for more than three hundred sols, and then, just at the right edge of the graph, bam!, it shoots up off the scale.

We're going to ask MGS, ODY, and maybe MEX if they have data that can add anything to our understanding of the situation. If not, maybe they'd like to gather some -- we have ground truth to offer them, making it potentially win-win. But one of the participants online says, "It would have to be a hell of a storm for the orbiters to see it." So in a sense, if we're lucky, that won't work out.

I remember something I read, pre-landing, about how Martian dust storms work. The dust traps heat like a blanket, making the local area warmer; this causes wind to rush in from colder areas, bringing more dust, which thickens the blanket and makes the area still warmer, and so on. In other words, once the winds get started, they can start a feedback loop that causes them to accelerate. So, lots of wind, and scattered debris nearby ....

"Hey," I ask, "will the wind blow this debris into the cameras or the wheels? Should we get the heck out of here -- maybe go back and hide in the crater? Maybe just find a way to get upwind?"

This idea causes some consternation, but the consensus is that there's no likely danger. Mars's air is so thin that the wind force is about the square root of what we see on Earth -- a 100km/hr wind there would be like a 10km/hr wind here. (Another interesting tidbit that comes out of this: Art mentions that the crater has its own weather system, a fascinating fact I'll have to remember to follow up on.)

One of the online experts also seems to think the local winds aren't significant anyway. "This dust could be just drifting in from a storm kilometers away," he says. "In fact, I'd put my money on it."

Still, Art wants someone to keep an eye on this. "Can we tell the wind direction?" he asks. People start chewing on the problem. "We could watch the debris move." "Watch a pile of dust?" "Make a pile of dust with the wheels and watch that."

I love working with smart people.

"Statistically," the disembodied voice offers, "there are two things working in your favor. First, MGS has been here, what, three Martian years or so, and has seen no global dust events in this season. That's consistent with what Viking saw. And second, tau will fall off faster than you saw from the post-landing storm. At this season, water ice will nucleate around the airborne dust and help scrub it."

Summary consensus from the meeting: we should act cautiously, but this is not a spacecraft-threatening event.

As the meeting is breaking up, Jim Erickson walks in and tries to discreetly take a seat in the back. (Jim's on vacation this week, so naturally he's here at work.) Art turns to him and says, "We drove your spacecraft into a storm."

Courtesy NASA/JPL/Cornell. The whole scene, from the sol before, in beautiful color. This is just magnificent, truly one of the signature images of the entire mission. It's a piece of a crashed spaceship on another world. OK, so it's our spaceship -- you think that makes it less impressive? (Follow the link for a full-sized image.)